The present invention relates to an improved gypsum-based substrate faced with a polymeric nonwoven sheet material, the gypsum-based substrate suited for use in construction materials such as wall panels, ceiling panels, floor underlayment and interior and exterior sheathing.
Gypsum board is traditionally manufactured by a continuous process. In the process, a gypsum slurry is first generated in a mechanical mixer by mixing calcium sulfate hemihydrate (also known as calcined gypsum), water, and other agents. These various additives are used in the gypsum slurry as set accelerators (such as ground gypsum, potassium sulphate), set retarders (such as diethylene triamine tetra acetic acid), water reducing agents (such as condensed naphthalene sulphonates), foaming agents (such as lauryl alcohol ether sulphates), liner bonding agents (such as starch), anti-burning agents (such as boric acid), glass fibers for improved physical properties and fire resistance, other agents to improve reaction to fire properties (such as clay), water proofing agents (such as wax or silicones), or other agents. The gypsum slurry is deposited on a paper sheet which has had each edge scored or creased to facilitate the folding of the edges to make a sidewall of height equal to board thickness and a further flap of width about 1 inch wide folded back over the board. An upper continuously advancing paper sheet is then laid over the gypsum slurry and the edges of the upper and lower sheets are pasted to each other using glue at the edges of the top and/or bottom sheet. The paper sheets and gypsum slurry are passed between parallel upper and lower forming plates or rolls in order to generate an integrated and continuous flat strip of unset gypsum sandwiched between the paper sheets that are known as facing or liners. This strip is conveyed over a series of continuous moving belts and rollers for a period of 2 to 5 minutes during which time the core begins to hydrate back to gypsum and hardens. During each transfer between belts and/or rolls, the strip is stressed in a way that can cause the paper facing to delaminate from the gypsum core if the adhesion between the gypsum core and the facing is not sufficient. Once the gypsum core has set sufficiently, the continuous strip is cut into shorter lengths or even individual boards or panels of prescribed length. Once again, it is important for there to be good adhesion between the paper sheets and the set, but still wet, gypsum core or the cutting action will pull the edges of the paper facing sheet away from the gypsum core. Good adhesion between the top and bottom paper sheet at the edges which are pasted with glue is also important here.
After the cutting step, the gypsum boards are separated and grouped through a series of belts and rollers and then flipped over before being fed into drying ovens or kilns where the boards are dried so as to evaporate excess water. The hydration from hemihydrate to gypsum must be essentially complete by this point, normally between 7 and 15 minutes after mixing. When the gypsum boards are accelerated, flipped and fed into the drying ovens, the boards are subjected to a variety of stresses that can cause the facing to peel away from the gypsum core of the boards unless there is good adhesion between the set (but still wet) gypsum core and the facing material. Inside the drying ovens, the boards are blown with hot drying air at speeds up to 4000 feet/minute which can cause further delamination of the paper facing if there is not good wet adhesion between the gypsum and the paper liners. If portions of the facing sheets delaminate from the gypsum core during drying in the oven, the liner can become entangled in the rollers and the gypsum crumbles as it dries, jamming the oven, which then requires the line to be shut down while the loose gypsum and liner is cleaned out of the ovens. Poor wet bond between liner and the gypsum core can also result in blisters due to delamination during the drying process. The gypsum boards are dried in the ovens for anywhere from 30 to 75 minutes. After the dried gypsum boards are removed from the ovens, the ends of the boards are trimmed off and the boards are cut to desired sizes. Good adhesion between the top and bottom paper sheet at the edges which are pasted with glue is also important throughout the board forming process as well as during use of the board.
Gypsum board has been the subject of numerous patents, such as U.S. Pat. No. 4,057,443, Canadian Patent No. 1,189,434, as well as co-pending U.S. patent application Ser. Nos. 09/512,921 and 09/513,097, all of which are incorporated herein by reference.
For years it has been recognized that high toughness and abuse resistance are desirable properties in gypsum-based board for use in buildings. High toughness and abuse resistance are here defined in terms of high initial modulus, high flexural strength corresponding to high-to-moderate initial modulus, high maximum flexural strength and high work-to-break. In addition to high toughness, it is desirable for gypsum board to have an abrasion resistant property in order to resist abuse. Further, it is desired to have gypsum board with some flexibility under load.
Standard gypsum boards are produced with a cellulosic paper liner. Paper has good wet adhesion with the gypsum slurry during board formation. It is believed that cellulose draws moisture from the slurry and pulls the slurry into close contact with the paper fibers. As the gypsum sets, there is some interlocking of the gypsum crystals with the paper fibers at the surface of the liner, as well as some chemical bonding between the wet gypsum matrix and the hydrophilic paper fibers. Paper does not allow the gypsum slurry to seep through during board forming, provides reasonable strength and a paintable surface to the finished gypsum board.
However, there are several disadvantages to the use of paper as a liner for gypsum board. Paper acts as a food source for mold and mildew, and it becomes especially weak and subject to delamination either directly from the gypsum core or between the layers of the multi-layer sheets when the paper becomes damp due to water leaks or high humidity.
In addition, standard paper-lined gypsum board has lower work-to-break and abrasion resistance than is needed for certain applications. Work-to-break (WTB) is defined as the force (or stress) required to break the sample times the distance (or strain) that the sample is deformed before failure. On a stress- strain curve, WTB is represented by the area under this stress-strain or breaking curve.
In use, paper-faced gypsum boards are generally coated with another material, such as specialty paint or wall coverings, in order to achieve high abrasion resistance. To overcome these durability problems, paper-faced board is frequently covered with a wall paper of hard sheet or plastic film when used in high traffic areas.
There are international and foreign building materials standards that also classify conventional gypsum boards in the combustible category. There have been efforts to make panel products from gypsum that can achieve noncombustible status. The weight fraction of 5-6% paper in standard paper-lined gypsum would most likely cause this building material to fail the test for combustibility as described in ASTM E136, were it not for the fact that some building materials, such as gypsum board, have been defined in section X1.2.3b of the standard as noncombustible, based on their composition and flame spread properties alone. U.S. Pat. No. 6,221,521 concerning gypsum boards made without liners describes how even these gypsum/fiber boards that are reinforced with internal cellulosic fibers instead of external paper liners are deemed combustible as tested by ASTM E136 because of the presence of more than 3-4% of organic fibers in the core of the board.
A normal paper liner contains about 170-220 g/m2 of cellulosic content. Similar standards exist in other countries and new European standards being implemented do not classify standard cellulosic paper-lined gypsum board as noncombustible, due to the calorific content of the surface liner.
As technology evolves regarding fire protection in buildings, consideration is being given to both building system fire resistance as well as a product""s reaction to fire. It is desirable to have a gypsum board that contributes as little fuel load as possible in a fire situation to improve the overall fire risk in buildings. It is beneficial for the liners used to make gypsum board to have a low calorific content in order to reduce the fuel load brought about through its use in the building.
Commercially available gypsum board products with liners other than cellulosic paper have been developed, an example being Dens-Glass(copyright) Gold exterior sheathing (available from Georgia-Pacific, Inc., Atlanta, Ga.). Dens-Glass(copyright) Gold exterior sheathing uses a glass mat in place of cellulosic paper liner. However, this product has relatively low WTB and low deflection and hence, is brittle. In addition, the surface of the Dens-Glass(copyright) Gold exterior sheathing is very different from standard cellulosic paper-lined gypsum board for interior use. For use in interior walls, it is desired to have a gypsum board with a surface similar to standard paper-lined gypsum board so that it can be painted and have a similar appearance as standard paper-lined board.
It has been a notorious problem with the standard paper-lined gypsum board that the paper liner peels off while removing wall paper. The most common technique for removing the old wall paper is to perforate the old wall paper by scoring and then wetting the perforated wall paper with water to loosen up the glue underneath the wall paper, which results in moist paper liner and hence, the paper liner becomes very susceptible to peeling when the wall paper is removed.
There have been attempts to substitute stronger and more durable synthetic sheet materials for the paper liners found in conventional gypsum board products. Canadian Patent No. 1,189,434 discloses gypsum panels made with a facing of a moisture vapor permeable spunbonded nonwoven material. Canadian Patent No. 1,189,434 discloses gypsum panels faced with Tyvek(copyright) spunbonded olefin sheet material. Tyvek(copyright) is a registered trademark of E.I. du Pont de Nemours and Company of Wilmington, Del. Tyvek(copyright) sheets are made by solution flash-spinning polyethylene to form fine plexifilamentary fibril structures that can be thermally bonded to form sheet material. The product of Canadian patent number 1,189,434 has several shortcomings. The product has been found to have poor adhesive bonding between the liner material and the gypsum slurry during the board manufacturing process. In addition, although the Tyvek(copyright) liner is as strong as paper in the machine direction (MD) and almost three times as strong in the cross direction (CD), the board strength is about one-third that of paper-lined standard gypsum board in the MD of the liner. In addition, the surface of the gypsum board is shiny and almost film-like smooth, which are characteristics of the Tyvek(copyright) sheet surface. Also, the melting point of Tyvek(copyright) sheet is quite low at 135xc2x0 C., and the sheet starts shrinking at temperatures close to 100xc2x0 C. This is a disadvantage because the drying ovens used in conventional gypsum board-making processes operate at temperatures well above 100xc2x0 C., usually above 150xc2x0 C.
It is desired to have gypsum board which would not sag or significantly lose its flexural strength when wet or in a high humidity environment. In addition, it is also desired to have abrasion resistant gypsum board. It is also desired to have gypsum board with high peel strength between the liner and the core. It would also be desirable to have good release properties between the liner and an overlying covering.
It is also desired to have a gypsum board free of ingredients that would act as nutrients for mold growth. Conventional gypsum board contains organic matter which provides food for fungi such as mold and mildew.
It is an object of the present invention to provide a gypsum board which provides the following product attributes: flexibility, high toughness, high surface stability against abrasion and peeling, resistance to liquid water and high humidity, fire resistance, mold resistance and paper-like surface.
In one embodiment, the present invention relates to a gypsum board comprising a gypsum core held between two sheets of porous, fibrous polymeric nonwoven liner, wherein the work-to-break of the gypsum board in the MD of the nonwoven liner at a strain of 0.75 inch is greater than 30 lb.-in.
In another embodiment, the invention also relates to a gypsum board having a work-to-break in the MD at a strain of 0.75 inch of greater than 60*X lb-in, where X is the thickness of the board in inches.
In another embodiment, the present invention is directed to a gypsum board comprising a gypsum core held between two sheets of porous, fibrous polymeric nonwoven liner, wherein the nonwoven liner has strip tensile in the machine direction of at least 35 lb./in., percent elongation at 1 lb. in the MD of less than 0.7%, percent elongation at 3 lb. in the MD of less than 1.5% and percent elongation-at-break in the MD of less than 100%, strip tensile in the CD of at least 12 lb./in., percent elongation at 1 lb. in the CD of less than 3.0%, percent elongation at 3 lb. in the CD of less than 7.0% and percent elongation-at-break in the CD of less than 300%.
In another embodiment, the present invention is directed to a gypsum board comprising a gypsum core held between two sheets of porous, fibrous polymeric nonwoven liner, wherein the nonwoven liner has strip tensile in the MD of at least 65 lb./in., percent elongation at 1 lb. in the MD of less than 0.5%, percent elongation at 3 lb. in the MD of less than 0.7% and percent elongation-at-break in the MD of less than 50%, strip tensile in the CD of at least 22 lb./in., percent elongation at 1 lb. in the CD of less than 1.5%, percent elongation at 3 lb. in the CD of less than 3.0% and percent elongation-at-break in the CD of less than 100%.